Conventionally, as an electrophotographic full-color image forming apparatus, there is known a tandem machine provided with four photoreceptor drums corresponding to four colors, that is, yellow, cyan, magenta, and black. The image forming apparatus is configured such that after an electrostatic latent image on the photoreceptor drum is developed for each color with toner, a toner image in each color is sequentially superimposed on a paper to obtain a full color image. The image forming apparatus is provided with an optical scanning apparatus that scans a beam light ray on a circumferential surface of each photoreceptor drum.
As the optical scanning apparatus, there is known an optical scanning apparatus configured such that first to fourth beam light rays emitted from a light source unit enter the same reflective surface of a rotating polygon mirror, with a different incidence angle. In the optical scanning apparatus, the first to fourth beam light rays reflected by the rotating polygon mirror are reflected by first to fourth reflective mirrors, respectively, and guided to the circumferential surface of the four photoreceptor drums.
In the optical scanning apparatus of this type, it is proposed a technology in which as a reflection position by the reflective mirror is closer to the rotating polygon mirror, an angle formed between a beam light ray and a beam light ray adjacent thereto is set larger.